Indeed, nail tattoos are mostly related to the nails of the crucifixion. But they are not all related to christianity.
The saying is 'tough as nails' and indeed, nail tattoos are often rough, solid and huge pieces of metal. The nail stands for destiny, and not its bright side. It is showing the scars, traumas and things we can't forget, yet we accept to live with. Nails in a skull are showing these negative things are coming with us until the end, or leading us to it. Indeed, these tattoos are related to the nails of the coffins. They are a little reminder of our fate and how we need to enjoy each second. But these pieces of hardware are also a symbol of ourselves. Receiving the strokes of life, and sometimes a bit twisted. Yet, we are not useless, just different... You could have many reasons to fill your sleeves and adorn your body with nail tattoos, and if you are looking for ideas, perhaps one of these examples nailed it!